The invention relates to a process for the stripping of paint from plastics parts such as used parts or reject parts with the aim of recycling the stripped plastic, in which the plastics parts are first mechanically comminuted to a pourable size, in which the plastic chips are circulated at elevated temperature together with an alkalified glycol, and in which after a certain treatment time the stripped plastics chips are separated from the stripping agent, flushed and then dried. British Patent Document GB-B 1 134 967 describes a process of this general type.
In recent years there has been a continual increase in the use of plastics in the automotive industry. In order to adapt the plastics, for example polycarbonates, even better to their use in the automotive sector, their surfaces are coated. Coatings are used to afford protection, for example against environmental effects, to bumpers and other plastics parts which form an integrated component in the body, and to integrate these parts visually into the color scheme of the motor vehicle. The recycling of the materials mentioned gives rise to quite specific problems. The colored layer of two-component paints breaks down mechanically when the plastics are recycled into small tinsel-like particles of paint which lead to an impairment of the recovered raw materials, and consequently this material can then only be processed into low-grade products. The particles of paint which remain behind cause, for example, a mechanical weakening of articles produced from recycled material, since again and again they interrupt an integral material bond and constitute sites where cracks can start, in other words micronotches. Articles made from recycled plastic which has not been stripped beforehand have only a very low tensile strength and notched impact strength and a likewise very low tear strength and elongation at break in comparison to virgin plastic of the same type. For this reason and, on the other hand, because stripping is a comparatively expensive process, the stripping of used plastics has generally been abandoned and used parts of this kind have been disposed of in other ways.
In this context there has been no lack of attempts to develop different processes for the stripping of plastics.
In particle blasting processes, the consumption of abrasive agent for the stripping operation is a concomitant phenomenon which on the one hand causes an unfavorable rise in the consumption of the material for reprocessing operation and on the other hand, by the incorporation or diffusion of fragments of abrasive agent into the base material, leads to impairment of the recycled material. Moreover, uniform or complete paint removal cannot be achieved. Furthermore, in cases in which each workpiece is treated as a whole, then these processes have to be carried out individually and locally, and therefore manually, so that it is not possible to employ these processes in the stripping of used plastics at an acceptable level of cost.
Water blasting processes are unsuitable for plastics parts because of the low density and weight of the latter, since the plastics parts move away under the pressure of the blasting jet. In addition, the angle of the jet is an important parameter in these processes and cannot always be maintained when the surfaces are sharply curved or structured.
In the so-called melt filtering process in which the non-plastifiable constituents such as, for example, the particles of paint, are filtered off from the plastified polymer melt during extrusion, the degree of filter fineness required for adequate removal of the fine particles of paint and the considerable filter area required for this and the resulting high filter-area load are disadvantageous. According to investigations by the applicant, the high extrusion temperature which is required for satisfactory flowability through the filter and the marked differential pressure both degrade the polymer matrix of the plastic. In an expanded process, i.e., with combined abrasive treatment and filtering of the polymer melt, a large proportion of the paint is first removed by way of a cone-type press and a pin mill, in order to reduce the amount of residual paint before filtering, the required filter area in the subsequent melt filtration and the filter load during this filtration is large. An objectionable feature of this process is again the damage to the plastic, both by the severe abrasive processes and by the melt filtration.
In a process for stripping the paint from complete bumper panels, which process is recommended by one vehicle manufacturer, these panels are softened in an oven and pressed flat before the paint is abrasively removed by means of brushes. A disadvantage of this process for the treatment of whole plastics parts is that noncomminuted and therefore bulky plastics parts have to be collected and treated centrally. Such bulky parts have a very low transportation density, which has an adverse effect on the transport costs and on the gross weight to be transported in comparison with the ecological benefits.
Although conceivable, chemical stripping with halogenated hydrocarbons (chlorofluorocarbons, chlorocarbons) is prohibited by law because of the environmental damage caused by these substances.
Another vehicle manufacturer specifics a process for the chemical stripping of plastics parts which uses an organic sulphide solution at from 80.degree. to 100.degree. C. This solution is not environmentally compatible. In addition, because of the ether and alcohol components there is the danger of autoignition or explosion, and therefore the possibility of simultaneous mechanical treatment can also be abandoned.
English Patent 1 134 967 describes a process for the recovery of film wastes in the photographic and film industry, which wastes consist of apparently expensive polyester, specifically and preferably poly-1,4-cyclohexanedimethylene terephthalate, thus making recovery worthwhile. The photosensitive layer can, for example, be softened in hot water and removed. However, before recovery of the base material of the film it is also necessary for an approximately 25 .mu.m, thin layer of adhesion promoter to be removed. And the layer of adhesion promoter consists in turn of a copolymeric vinylidene halide, preferably a copolymeric vinylidene chloride, which contains;:
from 30 to 98% by weight, preferably at least 35% by weight, of vinylidene halides, PA1 from 0.5 to 40% by weight, preferably from 10 to 15% by weight, of acrylonitriles and/or preferably an additional 1 to 12% by weight of alkyl esters of an acrylic, methacrylic or itaconic acid, and PA1 from 1 to 25% by weight of an acrylic, methacrylic or itaconic acid. PA1 complete removal of the paint from the plastics parts can be achieved; PA1 minimal damage to the plastics parts; PA1 minimal consumption of energy and processing auxiliaries; PA1 low process costs in comparison to the new value of the recycled material; PA1 only environmentally unobjectionable means are used; PA1 no ecologically objectionable emissions or residues; PA1 the process can be automated and can be operated continuously.
In order to remove this layer of adhesion promoter, the film material, which has been comminuted to chips having a lateral length of about 1 cm, is brought into contact with at least six times its weight, preferably from 10 to 50 times the quantity, of glycol which has been alkalified with sodium hydroxide or potassium hydroxide solution, at least 90.degree. C. and preferably from 145.degree. to 155.degree. C.; in other words, it is suspended in the alkalified glycol and this suspension is stirred or circulated for a length of time, preferably about 2 minutes, until at least the majority of the chips have been stripped. Subsequently the stripped chips and the glycol are separated from one another. According to the proposal, the glycol to be used is preferably ethylene glycol and specifically 1,4-cyclohexanedimethanol, in other words precisely that glycol which in its chemical structure corresponds to the specific polyester of the base material to be stripped. By means of the glycol which is recommended for the chemical stripping and is chemically related to the base material, this base material, at least at elevated temperature, is broken up or dissolved and in this way the bond with the layer to be removed is destroyed or broken, the layer to be removed coming off in sheets. In this process a partial breaking-up or dissolution of the base material is not merely tolerated but indeed is a specific aim. And indeed the upper limit recommended for the treatment temperature is a temperature at which approximately 5% by weight of the base material may be dissolved, but in any case less than about 200.degree. C. The proposed stripping process hails from a time (1966) when ecological considerations did not as yet play any decisive role in process design, but instead considerations of cost were the priority. Indeed, the use of ethylene glycol in such large amounts is highly objectionable from an ecological point of view, since this glycol is classified as "slightly toxic" on the toxicity scale.
An object of the invention is to specify a process which achieves complete removal of a coating from plastics parts with minimal damage to the plastics material while using environmentally compatible means and with a low consumption of material and energy, and which, at least in the case of plastics parts which are pourable, or have been precomminuted so as to render them pourable, can be carried out in a process which allows automation.
Starting from the process on which the generic art is based this object is achieved in accordance with the invention by means of a process wherein, in order to strip the coated plastics parts, they are circulated together with a stripping agent of an anhydrous, alkalified diethylene glycol or propylene glycol, in a mass with mutual abrasion of the plastics parts at at least 50.degree. C., the quantity of stripping agent added being from 1 to 10% of the volume of mass to be treated, so that the circulated parts are only wetted by the stripping agent. In especially preferred embodiments, the circulation is carried out at a temperature between 80.degree. C. and 100.degree. C. The chemical dissolution effect is brought about by means of an environmentally compatible two-component mixture of, on the one hand, diethylene glycol and/or propylene glycol and, on the other hand, potassium hydroxide and/or sodium hydroxide. Diethylene glycol is in water hazard class zero. After neutralization it is acceptable to pass on potassium hydroxide, as a dietary salt solution, for effluent treatment. In the course of stripping the polycarbonate is hydrolysed, a reaction which does not liberate any by-products classified as objectionable. The mechano-abrasive effect is designed so that no damage occurs to the plastics material but at the same time so that the paint is removed on all sides of the particle, irrespective of the specific particle form. Treatment in the reactor is carried out automatically and permits the stripping of various particle forms after appropriate precomminution. The reactor is closed during the treatment in order to save heat energy and to minimize the loss of chemicals by evaporation.
The advantages of the inventive embodiment of the stripping process are as follows:
The use of the process described is appropriate for all cases in which a polycarbonate or polycarbonate blend can be subjected to high-grade material recycling by stripping. This is the case in particular for the bumpers of the top car class, which consists of this material to the extent of more than 50% and more than 4 kg per part.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.